1. Field of the Invention
This invention relates generally to a system for determining the size of the air gap associated with a wheel speed sensor used in antilocking brake systems. More specifically, this invention relates to a device and methodology for monitoring the output of a wheel speed sensor associated with an antilocking brake system and for producing a D.C. voltage proportional to the air gap associated with that wheel and sensor such that the size of the air gap is determined.
2. Description of the Prior Art
Vehicle antilocking brake systems typically include a wheel speed sensor. A wheel speed sensor provides information about the speed of each wheel of the vehicle, for example, so the antilocking brake system electronic controller can determine whether or not a "lock up condition" exists. Conventional wheel speed sensors include a magnet and a coil that produce a magnetic field that interacts with an apparatus such as a tone wheel that rotates with the vehicle wheel. The tone wheel typically includes teeth or gearing that interacts with the magnetic field produced by the magnet and coil. The conditions of the magnetic field are indicative of the speed of the wheel. As the tone wheel rotates relative to the magnet and coil, the interaction of the teeth with the magnetic field cause changes within that field that affect the output signal of the wheel speed sensor. Therefore, the frequency of the sensor signal varies with the speed of the rotation of the wheel.
The output of the wheel speed sensor depends on the rotation of the wheel and the concomitant rotation of the tone wheel. The output of the sensor also depends on the distance between the tone wheel teeth and the magnet and coil in the wheel speed sensor housing. The distance between the sensor and the wheel is conventionally known as the air gap. Movement of the axle, associated with the wheel, relative to its bearing causes the tone wheel to move relative to the wheel speed sensor. Such movement of the tone wheel causes undesirable changes within the wheel speed sensor output, and in some cases introduces error into wheel speed detection. When the wheel axle moves relative to its bearing, the gap is changed between the wheel and the sensor. Information regarding an air gap can be useful in determining whether the wheel speed sensor output is reliable or whether undesirable error has been introduced through the change in the air gap. Therefore, monitoring an air gap in an antilocking brake system provides useful information.
One system used for monitoring an air gap requires the wheel speed sensor peak to peak voltage to be recorded. A peak to peak voltage is then measured with a ruler from the recording and the air gap is calculated from this information and knowledge of the wheel speed sensor sensitivity. Another air gap detector includes an active integrator that converts the wheel speed sensor output signal to a constant amplitude signal. This A.C. voltage signal is then converted and calculations must be performed to actually measure an air gap. These systems are typically cumbersome and include time consuming procedures and introduce potential errors into air gap measurement.
This invention avoids the cumbersome apparatus and procedures required in using the conventional devices described above. Further, this invention seeks to provide an air gap signal that is directly proportional to the air gap within an antilocking brake system wheel assembly which may be caused by relatively rough road conditions, for example.